Controller and control method of belt type continuously variable transmission

ABSTRACT

A control device for a belt type continuously variable transmission device includes primary And secondary pulleys and a belt and controls gear ratio based on a running radius of the belt on a pulley by controlling primary and secondary oil pressures. The transmission includes a belt slip control unit and a belt slip control permission determining unit. The belt slip control unit performs such control as to oscillate the secondary oil pressure, estimates a belt slip condition by monitoring the phase difference between an oscillation component included in an actual secondary oil pressure and an oscillation component included in an actual gear ration, and then reduces the actual secondary oil pressure to maintain a predetermined belt slip condition. The belt slip control permission determining unit permits belt slip control when a transmission rate as a change rate of the gear ratio is less than a predetermined value.

TECHNICAL FIELD

The present invention relates to a device and a method for controlling abelt type continuously variable transmission to perform a belt slipcontrol in which a belt wrapped around pulleys is slipped at apredetermined slip rate.

BACKGROUND ART

A known belt type continuously variable transmission controller isconfigured to perform a belt slip control in which an actual secondaryhydraulic pressure is reduced from one during a normal control to slip abelt wrapped around pulleys at a predetermined slip rate by thefollowing steps:

(a) superimposing a predetermined sine wave on a command secondaryhydraulic pressure or oscillating the command secondary hydraulicpressure, and

(b) performing the belt slip control by controlling the actual secondaryhydraulic pressure on the basis of a multiplier of an oscillationcomponent included in the actual secondary hydraulic pressure and anoscillation component included in an actual gear ratio. This eliminatesthe necessity for directly detecting the belt slip rate and therebyfacilitates the belt slip control (see Patent Document 1, for example).

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: WO 2009/007450 A2 (PCT/EP2008/059092)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, there is a problem with such a conventional belt typecontinuously variable transmission controller in that at a hightransmission speed, it cannot discriminate (distinguish) between acommand gear ratio component and an oscillation component for detectinga slip rate from a variation characteristic of the actual gear ratio sothat it is inappropriate to perform the belt slip control based on themultiplier of an oscillation component included in the actual secondaryhydraulic pressure and that included in an actual gear ratio.Accordingly, a large belt slip may occur depending on the magnitude of atorque input to the belt type continuously variable transmission.

In view of solving the above problem, the present invention aims toprovide control device and method for a belt type continuously variabletransmission which can reduce driving energy consumption due to adecrease in belt friction when the estimated accuracy of a belt slipcondition is high as well as can prevent the belt from greatly slippingduring the belt slip control when the estimated accuracy of a belt slipcondition is low.

Means to Solve the Problem

To attain the above object, a control device for a belt typecontinuously variable transmission according to the present inventionincludes a primary pulley for receiving an input from a drive source, asecondary pulley for providing an output to a drive wheel, and a beltwrapped around the primary pulley and the secondary pulley, to control agear ratio determined by a running radius ratio of the belt on thepulleys by controlling a primary hydraulic pressure to the primarypulley and a secondary hydraulic pressure to the secondary pulley. Thedevice further comprises a belt slip control means configured tooscillate the secondary hydraulic pressure and monitor a phasedifference between an oscillation component included in an actualsecondary hydraulic pressure and an oscillation component included in anactual gear ratio to estimate a belt slip condition, and control theactual secondary hydraulic pressure to decrease on the basis of theestimation to maintain a predetermined belt slip condition, and a beltslip control permission determining means configured to permit the beltslip control means to perform a belt slip control when a change speed ofthe gear ratio of the belt type continuously variable transmission isless than a predetermined value at which the oscillation component dueto the oscillation included in an actual gear ratio and a variation onthe actual gear ratio can be separated.

Effects of the Invention

Thus, according to the control device for the belt type continuouslyvariable transmission, the belt slip control permission determiningmeans permits the belt slip control means to perform a belt slip controlwhen the change speed of the gear ratio is less than a predeterminedvalue. That is, under the belt slip control, since the belt slipcondition is estimated using the oscillation component included in theactual gear ratio by the oscillation, the change speed of the gear ratioaffects the extraction of the oscillation component. When the changespeed of the gear ratio is less than the predetermined value, avariation in the gear ratio due to the transmission and the oscillationcomponent due to the oscillation can be separated. Meanwhile, when thechange speed of the gear ratio exceeds the predetermined value, theoscillation component included in the actual gear ratio disappears sothat the gear ratio variation due to the transmission and theoscillation component due to the oscillation cannot be separated. To thecontrary, when the change speed of the gear ratio with a high estimatedaccuracy of the belt slip condition is lower than the predeterminedvalue, the belt slip control is permitted, reducing belt friction owingto a reduction in the pulley hydraulic pressure and reducing a driveload on the transmission mechanism. Meanwhile, when the change speed ofthe gear ratio with a high estimated accuracy of the belt slip conditionexceeds the predetermined value, the belt slip control is not permitted,preventing the belt from greatly slipping as in the case where the beltslip control is permitted irrespective of the change speed of the gearratio. This makes it possible to reduce driving energy consumption dueto a decrease in belt friction when the estimated accuracy of a beltslip condition is high as well as can prevent the belt from greatlyslipping during the belt slip control when the estimated accuracy of abelt slip condition is low.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the entire system of a drive system and a control system ofa vehicle incorporating a belt type continuously variable transmissionapplied with a control device and method according to a firstembodiment.

FIG. 2 is a perspective view of the belt type continuously variabletransmission mechanism applied with the control device and methodaccording to the first embodiment.

FIG. 3 is a perspective view of a part of a belt of the belt typecontinuously variable transmission mechanism applied with the controldevice and method according to the first embodiment.

FIG. 4 is a control block diagram of the line pressure control andsecondary hydraulic pressure control (normal control/belt slip control)executed by a CVT control unit 8 according to the first embodiment.

FIG. 5 is a basic flowchart for a switching process between the normalcontrol and the belt slip control (=BSC) over the secondary hydraulicpressure executed by the CVT control unit 8 according to the firstembodiment.

FIG. 6 is a flowchart for the entire belt slip control process executedby the CVT control unit 8 according to the first embodiment.

FIG. 7 is a flowchart for the torque limit process of the belt slipcontrol process executed by the CVT control unit 8 according to thefirst embodiment.

FIG. 8 is a flowchart for the secondary hydraulic pressure oscillationand correction process of the belt slip control process executed by theCVT control unit 8 according to the first embodiment.

FIG. 9 is a flowchart for a returning process from the belt slip controlto the normal control executed by the CVT control unit 8 according tothe first embodiment.

FIG. 10 is a flowchart for the torque limit process of the returningprocess to the normal control executed by the CVT control unit 8according to the first embodiment.

FIG. 11 is a flowchart for the transmission restricting process of thereturning process to the normal control executed by the CVT control unit8 according to the first embodiment.

FIG. 12 is a timing chart of an actual gear ratio characteristic and atarget gear ratio characteristic when transmission is changed at a smalltransmission change rate during the belt slip control.

FIG. 13 is a timing chart of an actual gear ratio characteristic and atarget gear ratio characteristic when transmission is changed at a largetransmission change rate during the belt slip control.

FIG. 14 shows how to decide a predetermined value as an upper limitthreshold for the gear ratio change rate, and Graph A shows thefrequency characteristics of a command gear ratio component and of anoscillation component, and Graph B shows a transmission condition whenthe transmission range and transmission time constant are changed.

FIG. 15 is a timing chart of the respective characteristics of BSCoperation flag, SEC pressure F/B inhibition flag, accelerator opening,vehicle speed, engine torque, Ratio, SEC hydraulic pressure. SEC_SOL,current correction amount, and phase difference between SEC pressureoscillation and Ratio oscillation in a traveling scene during a controlshift from the normal control, belt slip control, returning control tothe normal control.

FIG. 16 is a timing chart of the torque limit control for illustratingthe returning control from the belt slip control to the normal control.

EMBODIMENTS OF DESCRIPTION

Hereinafter, the best mode to carry out the control device and methodfor a belt type continuously variable transmission will be describedusing a first embodiment with reference to the accompanying drawings.

First Embodiment

First, the structure of the device is described. FIG. 1 shows the entiresystem of a drive system and a control system of a vehicle incorporatinga belt type continuously variable transmission applied with a controldevice and method according to the first embodiment. FIG. 2 is aperspective view of the belt type continuously variable transmissionmechanism applied with the control device and method according to thefirst embodiment. FIG. 3 is a perspective view of a part of a belt of abelt type continuously variable transmission mechanism applied with thecontrol device and method according to the first embodiment. In thefollowing the system structures are described with reference to FIGS. 1to 3.

In FIG. 1 the drive system of a vehicle incorporating a belt typecontinuously variable transmission comprises an engine 1, a torqueconverter 2, a forward/backward drive switch mechanism 3, a belt typecontinuously variable transmission mechanism 4, a final reductionmechanism 5 and drive wheels 6, 6.

The output torque of the engine 1 is controllable by an engine controlsignal supplied from the exterior in addition to by a driver'sacceleration operation. The engine 1 includes an output torque controlactuator 10 to control the output torque by a throttle valveopening/closing operation, a fuel cut operation and else.

The torque converter 2 is a startup element with a torque increasingfunction and includes a lockup clutch 20 to be able to directly connectan engine output shaft 11 (=torque converter input shaft) and a torqueconverter output shaft 21. The torque converter 2 is comprised of aturbine runner 23 connected with the engine output shaft 11 via aconverter housing 22, an impeller pump 24 connected with the torqueconverter output shaft 21, and a stator 26 provided via a one-way clutch25.

The forward/backward drive switch mechanism 3 is to switch a rotarydirection input to the belt type continuously variable transmissionmechanism 4 between a normal rotary direction during forward travelingand a reverse rotary direction during backward traveling. Theforward/backward switch mechanism 3 includes a double pinion planetarygear 30, a forward clutch 31, and a backward brake 32. A sun gear of thedouble pinion planetary gear 30 is connected with the torque converteroutput shaft 21 and a carrier thereof is connected with a transmissioninput shaft 40. The forward clutch 31 is fastened during a backwardtraveling to fix a ring gear of the double pinion planetary gear 30 tothe case.

The belt type continuously variable transmission mechanism 4 has acontinuously variable transmission function to steplessly vary the gearratio by changing a belt contact radius. The gear ratio is a ratio ofthe input rotation rate of the transmission input shall 40 and theoutput rotation rate of the transmission output shaft 41. The belt typecontinuously variable transmission mechanism 4 includes a primary pulley42, a secondary pulley 43, and a belt 44. The primary pulley 42 is madeup of a fixed pulley 42 a and a slide pulley 42 b. The slide pulley 42 bis slid by primary hydraulic pressure introduced into a primaryhydraulic pressure chamber 45. The secondary pulley 43 is made up of afixed pulley 43 a and a slide pulley 43 b. The slide pulley 43 h is slidby primary hydraulic pressure introduced into a secondary hydraulicpressure chamber 46. The belt 44 as shown in FIG. 2 is wrapped aroundV-form sheave faces 42 c, 42 d of the primary pulley 42 and V-formsheave faces 43 c, 43 d of the secondary pulley 43. In FIG. 3 the belt44 is formed of two laminated rings 44 a, 44 a of which a large numberof rings are layered from inside to outside as well as a large number ofelements 44 h of press-cut plates placed between the two laminated rings44 a, 44 a and connected with each other in a ring-form. The elements 44b each includes, at both sides, flank faces 44 c, 44 c to contact withthe sheave faces 42 c, 42 d of the primary pulley 42 and the sheavefaces 43 c, 43 d of the secondary pulley 43.

The final reduction mechanism 5 decelerates the transmission outputrotation from the transmission output shaft 41 of the belt typecontinuously variable transmission mechanism 4 and provides adifferential function thereto to transmit it to the right and left drivewheels 6, 6. The final reduction mechanism 5 is interposed among thetransmission output shaft 41, an idler shaft 50, right and left driveshafts 51, 51, and includes a first gear 52, a second gear 53, a thirdgear 54, and a fourth gear 55 with a deceleration function and a geardifferential gear 56 with a differential function.

The control system of the belt type continuously variable transmissioncomprises a transmission hydraulic pressure control unit 7 and a CVTcontrol unit 8, as shown in FIG. 1.

The transmission hydraulic pressure control unit 7 is a hydraulicpressure control unit to produce primary hydraulic, pressure introducedinto the primary hydraulic pressure chamber 45 and secondary hydraulicpressure introduced into the secondary hydraulic pressure chamber 46.The transmission hydraulic pressure control unit 7 comprises an oil pump70, a regulator valve 71, a line pressure solenoid 72, a transmissioncontrol valve 73, a decompression valve 74, a secondary hydraulicpressure solenoid 75, a servo link 76, a transmission command valve 77,and a step motor 78.

The regulator valve 71 uses discharged pressure from the oil pump 70 asa pressure source to adjust line pressure PL. The regulator valve 71includes the line pressure solenoid 72 to adjust the pressure of oilfrom the oil pump 70 to a predetermined line pressure PL in response toa command from the CVT control unit 8.

The transmission control valve 73 uses the line pressure PL produced bythe regulator valve 71 as a pressure source to adjust the primaryhydraulic pressure introduced into the primary hydraulic pressurechamber 45. A spool 73 a of the transmission control valve 73 isconnected with the servo link 76 constituting a mechanical feedbackmechanism and the transmission command valve 77 connected with one endof the servo link 76 is driven by the step motor 78 so that thetransmission control valve receives feedback of a slide position (actualpulley ratio) from the slide pulley 42 b of the primary pulley 42connected with the other end of the servo link 76. That is, duringtransmission, when the step motor 78 is driven in response to a commandfrom the CVT control unit 8, the spool 73 a of the transmission controlvalve 73 is changed in position to supply/discharge the line pressurePL, to/from the primary hydraulic pressure chamber 45 to adjust theprimary hydraulic pressure to acquire a target gear ratio commanded atthe drive position of the step motor 78. Upon completion of thetransmission, the spool 73 a is held at a closed position in response toa displacement of the servo link 76.

The decompression valve 74 uses the line pressure PL produced by theregulator valve 71 as a pressure source to adjust the secondaryhydraulic pressure introduced into the secondary hydraulic pressurechamber 46 by decompression. The decompression valve 74 comprises thesecondary hydraulic pressure solenoid 75 to decompress the line pressurePL to a command secondary hydraulic pressure in accordance with acommand from the CVT control unit 8.

The CVT control unit 8 is configured to perform various control such asa gear ratio control to output to the step motor 78 a control command toacquire a target gear ratio in accordance with vehicle speed, throttleopening level and else, a line pressure control to output to the linepressure solenoid 72 a control command to acquire a target line pressurein accordance with the throttle opening level or else, a secondaryhydraulic pressure control to output to the secondary hydraulic pressuresolenoid 75 a control command to acquire a target secondary pulleythrust in accordance with transmission input torque or else, a forwardand backward switch control to control the fastening and release of theforward clutch 31 and backward brake 32, and a lockup control to controlfastening and release of the lockup clutch 20. The CVT control unit 8receives various sensor information and switch information from aprimary rotation sensor 80, a secondary rotation sensor 81, a secondaryhydraulic pressure sensor 82, an oil temperature sensor 83, an inhibitorswitch 84, a brake switch 85, an accelerator opening sensor 86, andother sensors and switches 87. Further, it receives torque informationfrom an engine control unit 88 and outputs a torque request to theengine control unit 88.

FIG. 4 is a control block diagram of the line pressure control andsecondary hydraulic pressure control (normal control/belt slip control)executed by the CVT control unit 8 according to the first embodiment.

The hydraulic pressure control system of the CVT control unit 8 in thefirst embodiment comprises a basic hydraulic pressure calculator 90, aline pressure controller 91, a secondary hydraulic pressure controller92, a sine wave oscillation controller 93, and a secondary hydraulicpressure corrector 94, as shown in FIG. 4.

The basic hydraulic pressure calculator 90 includes an input torquecalculator 90 a to calculate transmission input torque on the basis ofthe torque information (engine rotary rate, fuel injection time and thelike) from the engine control unit 88 (FIG. 1), a basic secondary thrustcalculator 90 b to calculate a basic secondary thrust (belt clamp forcenecessary for the secondary pulley 43) from the transmission inputtorque obtained by the input torque calculator 90 a, a transmissionrequired thrust difference calculator 90 c to calculate a thrustdifference required for transmission (a difference in belt clamp forcebetween the primary and secondary pulleys 42, 43), a corrector 90 d tocorrect the calculated basic secondary thrust on the basis of therequired thrust difference for transmission, and a secondary hydraulicpressure converter 90 e to covert the corrected secondary thrust to atarget secondary hydraulic pressure. It further includes a basic primarythrust calculator 90 f to calculate a basic primary thrust (belt clampforce required for the primary pulley 42) from the transmission inputtorque calculated by the input torque calculator 90 a, a corrector 90 gto correct the calculated basic primary thrust on the basis of therequired thrust difference for transmission calculated by thetransmission required thrust difference calculator 90 c, and a primaryhydraulic pressure converter 90 h to convert the corrected primarythrust to a target primary hydraulic pressure.

The line pressure controller 91 includes a target line pressuredeterminer 91 a to compare the target primary hydraulic pressure outputfrom the primary hydraulic pressure converter 90 h with the commandsecondary hydraulic pressure output from the secondary hydraulicpressure controller 92, and set the target line pressure to the targetprimary hydraulic pressure when the target primary hydraulicpressure≧the command secondary hydraulic pressure and set the targetline pressure to the secondary hydraulic pressure when the targetprimary hydraulic pressure≦the command secondary hydraulic pressure, anda hydraulic pressure-current converter 91 b to convert the target linepressure determined by the target line pressure determiner 91 a to acurrent value applied to the solenoid and output a command current valueconverted to the line pressure solenoid 72 of the regulator valve 71.

In the normal control the secondary hydraulic pressure controller 92performs the feedback control using the actual secondary hydraulicpressure detected by the secondary hydraulic pressure sensor 82 toacquire a command secondary hydraulic pressure while in the belt slipcontrol it performs open control without using the actual secondaryhydraulic pressure to acquire the command secondary hydraulic pressure.It includes a low pass filter 92 a through which the target secondaryhydraulic pressure from the secondary hydraulic pressure converter 90 eis filtered, a deviation calculator 92 b to calculate a deviationbetween the actual secondary hydraulic pressure and the target secondaryhydraulic pressure, a zero deviation setter 92 c to set the deviation tozero, a deviation switch 92 d to selectively switch between thecalculated deviation and zero deviation, and an integrated gaindeterminer 92 e to determine an integrated gain from oil temperature.Further, it includes a multiplier 92 f to multiply the integrated gainfrom the integrated gain determiner 92 e and the deviation from thedeviation switch 92 d, an integrator 92 g to integrate an FB integrationcontrol amount from the multiplier 92 f, an adder 92 h to add theintegrated FB integration control amount to the target secondaryhydraulic pressure from the secondary hydraulic pressure converter 90 e,and a limiter 92 i to set upper and lower limits to the added value toobtain the command secondary hydraulic pressure (referred to basicsecondary hydraulic pressure in the belt slip control). Further, itincludes an oscillation adder 92 j to add a sine wave oscillationcommand to the basic secondary hydraulic pressure in the belt slipcontrol, a hydraulic pressure corrector 92 k to correct the oscillatedbasic secondary hydraulic pressure by a secondary hydraulic pressurecorrection amount to the command secondary hydraulic pressure, and ahydraulic pressure-current converter 92 m to convert the commandsecondary hydraulic pressure into a current value applied to thesolenoid to output a command current value converted to the secondaryhydraulic pressure solenoid 75. Note that the deviation switch 92 d isconfigured to select the calculated deviation when a BSC operation flagis 0 (during the normal control) and select the zero deviation when theBSC operation flag is 1 (during the belt slip control).

The sine wave oscillation controller 93 includes a sine wave oscillator93 a to decide an oscillation frequency and an oscillation amplitudesuitable for the belt slip control and apply sine wave hydraulicpressure oscillation in accordance with the decided frequency andamplitude, a zero oscillation setter 93 b to apply no sine wavehydraulic pressure oscillation, and an oscillation switch 93 c toselectively switch between the hydraulic pressure oscillation and zerooscillation. Note that the oscillation switch 93 c is configured toselect the zero oscillation when the BSC operation flag is 0 (during thenormal control) and select the sine wave hydraulic pressure oscillationwhen the BSC operation flag is 1 (during the belt slip control).

The secondary hydraulic pressure corrector 94 includes an actual gearratio calculator 94 a to calculate an actual gear ratio Ratio from aratio of the primary rotary rate Npri of the primary rotation sensor 80and the secondary rotary rate Nsec of the secondary rotation sensor 81,a first bandpass filter 94 b to extract an oscillation component from asignal representing the actual secondary hydraulic pressure Psecobtained with the secondary hydraulic pressure sensor 82, and a secondbandpass filter 94 c to extract an oscillation component from thecalculated data by the actual gear ratio calculator 94 a. It furtherincludes a multiplier 94 d to multiply the oscillation componentsextracted by both bandpass filters 94 b, 94 c, a low pass filter 94 e toextract phase difference information from the multiplication result, asecondary hydraulic pressure correction amount determiner 94 f todetermine a secondary hydraulic pressure correction amount on the basisof the phase difference information from the low pass filter 94 e, azero correction amount setter 94 g to set the secondary hydraulicpressure correction amount to zero, and a correction amount switch 94 hto selectively switch between the secondary hydraulic pressurecorrection amount and the zero correction amount. Note that thecorrection amount switch 94 h is configured to select the zerocorrection amount when the BSC operation flag is 0 (during the normalcontrol) and select the secondary hydraulic pressure correction amountwhen the BSC operation flag is 1 (during the belt slip control).

FIG. 5 is a basic flowchart for a switching process between the normalcontrol and the belt slip control (=BSC) over the secondary hydraulicpressure executed by the CVT control unit 8 according to the firstembodiment. In the following the respective steps in FIG. 5 aredescribed.

In step S1 following a startup by turning-on of the key, thedetermination on non-BSC permission in step S2 or normal controlreturning process in step S5, the belt type continuously variabletransmission mechanism 4 is normally controlled, and then the flowproceeds to step S2. During the normal control, the BSC operation flagis set to zero.

In step S2 following the normal control in step S1, a determination ismade on whether or not all of the following BSC permission conditionsare satisfied. At the result being YES (all the BSC permissionconditions satisfied), the flow proceeding to step S3, the belt slipcontrol (BSC) is performed. At the result being NO (any of the BSCpermission conditions unsatisfied), the flow returning to step S1, thenormal control is continued. An example of the BSC permission conditionsis as follows:

(1) The transmitted torque capacity of the belt type continuouslyvariable transmission mechanism 4 is stable (a change rate of thetransmitted torque capacity is small).

This condition (1) is determined by satisfaction of the following twoconditions, for example.

-   a. |command torque change rate| < predetermined value-   b. |command gear ratio change rate| < predetermined value    (2) The estimated accuracy of the input torque to the primary pulley    42 is within a reliable range.    This condition (2) is for example determined on the basis of the    torque information (estimated engine torque) from the engine control    unit 88, the lockup state of the torque converter 2, the operation    state of a brake pedal, a range position and the like.    (3) The permitted conditions in the above (1) (2) are continued for    a predetermined length of time.    In step S2 whether or not the above conditions (1), (2), (3) are all    satisfied is determined.

In step S3 following the BSC permission determination in step S2 or theBSC continuation determination in step S4, the belt slip control (FIG. 6to FIG. 8) is performed to reduce an input to the belt 44 of the belttype continuously variable transmission mechanism 4 and maintain thebelt 44 in an appropriate slip state without slippage. Then, the flowproceeds to step S4. During the belt slip control the operation flag isset to 1.

In step S4 following the belt slip control in step S3, a determinationis made on whether or not all of the following BSC continuationconditions are satisfied. At the result being YES (all the BSCcontinuation conditions satisfied), the flow returning to step S3, thebelt slip control (BSC) is continued. At the result being NO (any of theBSC continuation conditions unsatisfied), the flow proceeds to step S5,and the normal control returning process is performed.

An example of the BSC continuation conditions is as follows:

(1) The transmitted torque capacity of the belt type continuouslyvariable transmission mechanism 4 is stable (a change rate of thetransmitted torque capacity is small).

This condition (1) is determined by satisfaction of the following twoconditions, for example.

-   a. |command torque change rate| < predetermined value-   b. |command gear ratio change rate| < predetermined vale    (2) The estimated accuracy of the input torque to the primary pulley    42 is within a reliable

This condition (2) is for example determined on the basis of the torqueinformation (estimated engine torque) from the engine control unit 88,the lockup state of the torque converter 2, the operation state of abrake pedal, a range position and the like. Whether or not the aboveconditions (1), (2) are both satisfied is determined. That is, adifference between the BSC permission conditions and the BSCcontinuation conditions is in that the BSC continuation conditionsexclude the continuation condition (3) of the BSC permission conditions.

In step S5 following a determination that any of the BSC continuationconditions is unsatisfied, the normal control returning process (FIG. 9to FIG. 11) is performed to prevent the belt 4 from slipping when thebelt slip control is returned to the normal control. Upon completion ofthe process, the flow returns to step S1 and shills to the normalcontrol.

FIG. 6 is a flowchart for the entire belt slip control process executedby the CVT control unit 8 according to the first embodiment. FIG. 7 is aflowchart for the torque limit process of the belt slip control processexecuted by the CVT control unit 8 according to the first embodiment.FIG. 8 is a flowchart for the secondary hydraulic pressure oscillationand correction process of the belt slip control process executed by theCVT control unit 8 according to the first embodiment.

First, as apparent from FIG. 6, during the belt slip control in whichthe BSC permission determination and the BSC continuation determinationare continued, a feedback control inhibition process (step S31) in whichthe command secondary hydraulic pressure is obtained using the actualsecondary hydraulic pressure, a torque limit process (step S32) as apreparation for returning to the normal control, and a secondaryhydraulic pressure oscillation and correction process (step S33) for thebelt slip control are concurrently performed.

In step S31 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thefeedback control under which the command secondary hydraulic pressure isobtained using the actual secondary hydraulic pressure detected by thesecondary hydraulic pressure sensor 82 is inhibited. That is, forobtaining the command secondary hydraulic pressure, the feedback controlduring the normal control is inhibited and switched to the open controlof the belt slip control using the zero deviation. Then, when the beltslip control is shifted to the normal control, the feedback controlreturns again.

In step S32 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thetorque limit process in FIG. 7 is performed. In step S321 of theflowchart in FIG. 7 a “torque limit request from the belt slip control”is defined to be the driver request torque.

In step S33 during the belt slip control in which the BSC permissiondetermination and the BSC continuation determination are continued, thesecondary hydraulic pressure is oscillated and corrected in FIG. 8. Inthe following the steps of the flowchart in FIG. 8 are described.

In step S331 the command secondary hydraulic pressure is oscillated.That is, the sine wave hydraulic pressure with predetermined amplitudeand predetermined frequency is superimposed on the command secondaryhydraulic pressure. The flow proceeds to step S332.

In step S332 following the oscillation of the command secondaryhydraulic pressure in step S331, the actual secondary hydraulic pressureis detected with the secondary hydraulic pressure sensor 82 to detectthe actual gear ratio by calculation based on information on the rotaryrates from the primary rotation sensor 80 and the secondary rotationsensor 81. The flow proceeds to step S333.

In step S333 following the detection of the actual secondary hydraulicpressure and the actual gear ratio in step S332, the actual secondaryhydraulic pressure and the gear ratio are each subjected to the bandpassfilter process to extract their respective oscillation components (sinewave) and multiply them. Then, the multiplied value is subjected to thelow pass filter process and converted to a value expressed by amplitudeand a phase difference θ (cosine wave) between the oscillation of theactual secondary hydraulic pressure and that of the actual gear ratio.The flow proceeds to step S334. Herein, where A is the amplitude of theactual secondary hydraulic pressure and B is the amplitude of the actualgear ratio, the oscillation of the actual secondary hydraulic pressureis expressed by the formula (1): A sin ωt. The oscillation of the actualgear ratio is expressed by the formula (2): B sin (ωt+θ). The formulas(1) and (2) are multiplied, and using the following product sum formula(3):sin α sin β=−½{ cos(α+β)−cos(α−β)}the following formula (4):A sin ωt×B sin(ωt+θ)=(½)AB cos θ−(½)AB cos(2ωt+θ)is obtained.In the formula (4), (½)AB cos(2ωt+θ) as the double component of theoscillation frequency is reduced through the low pass filter so that theformula (4) becomes the following formula (5):A sin ωt×B sin(ωt+θ)≈(½)Ab cos θThus, it can be expressed by the formula of the phase difference θ inthe oscillation between the actual secondary hydraulic pressure and theactual gear ratio.

In step S334 following the calculation of the phase difference θ in theoscillation between the actual secondary hydraulic pressure and theactual gear ratio, a determination is made on whether or not the phasedifference θ is such that 0≦phase difference θ<predetermined value at 1(micro slip range). At the result being YES (0≦phase differenceθ<predetermined value at 1), the flow proceeds to step S335 while at theresult being NO (predetermined value at 1≦phase difference θ), the flowproceeds to step S336.

In step S335 following the determination on 0≦phase differenceθ<predetermined value at 1 (micro slip range) in step S334, thesecondary hydraulic pressure correction amount is set to −Δpsec. Theflow proceeds to step S339.

In step S336 following the determination on predetermined value at1≦phase difference θ in step S334, a determination is made on whether ornot the phase difference θ is such that predetermined value at 1≦phasedifference θ<predetermined value at 2 (target slip range). At the resultbeing YES (predetermined value at 1≦phase difference θ<predeterminedvalue at 2), the flow proceeds to step S337 while at the result being NO(predetermined value at 2≦phase difference θ), the flow proceeds to stepS338.

In step S337 following the determination on predetermined value at1≦phase difference θ<predetermined value at 2 (target slip range) instep S336, the secondary hydraulic pressure correction amount is set tozero and the flow proceeds to step S339.

In step S338 following the determination on predetermined value at2≦phase difference θ (micro/macro slip transition range) in step S336,the secondary hydraulic pressure correction amount is set to +ΔPsec andthe flow proceeds to step S339.

In step S339 following the setting of the secondary hydraulic pressurecorrection amounts in steps S335, S337, S338, the command secondaryhydraulic pressure is set to the value of the basic secondary hydraulicpressure+secondary hydraulic pressure correction amount. Then, the flowends.

FIG. 9 is a flowchart for a returning process from the belt slip controlto the normal control executed by the CVT control unit 8 according tothe first embodiment. FIG. 10 is a flowchart for the torque limitprocess of the returning process to the normal control executed by theCVT control unit 8 according to the first embodiment. FIG. 11 is aflowchart for the transmission restricting process of the returningprocess to the normal control executed by the CVT control unit 8according to the first embodiment.

First, as apparent from FIG. 9, while the normal control is returnedfrom the belt slip control from the BSC continuation termination to thestart of the normal control, a feedback control returning process (stepS51) in which the command secondary hydraulic pressure is obtained usingthe actual secondary hydraulic pressure, a torque limit process (stepS52) as a preparation for returning to the normal control, anoscillation and correction secondary hydraulic pressure resettingprocess (step S53) for the belt slip control, and a transmissionrestricting process (step S54) in which the transmission speed as thechange speed of the gear ratio is restricted are concurrently performed.

In step S51, while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the feedback control in which the command secondary hydraulicpressure is obtained using the actual secondary hydraulic pressuredetected by the secondary hydraulic pressure sensor 82 is returned.

In step S52 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the torque limit process as a preparation for returning to thenormal control in FIG. 10 is performed.

In step S53 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the secondary hydraulic pressure oscillation and correction inFIG. 8 is reset to wait for the normal control.

In step S54 while the normal control is returned from the belt slipcontrol from the BSC continuation termination to the start of the normalcontrol, the transmission restricting process in which the transmissionspeed is restricted in FIG. 11 is performed.

In the following the steps of the flowchart showing the torque limitprocess in FIG. 10 are described. The key point of this torque limitprocess is to switch the controls on the basis of a magnitude relationamong the three values of driver request torque, torque limit requestfrom the BSC, and torque capacity (calculated torque capacity). Herein,the driver request torque refers to an engine torque requested by adriver, torque limit request from the BSC refers to torque limit amountshown in the phases (2), (3) in FIG. 16. Torque capacity is generally anallowable designed torque capacity and set to a value higher than thedriver request torque by a margin with mechanical variation of the belttype continuously variable transmission mechanism 4 taken intoconsideration, for the purpose of preventing the belt slip. Herein, theactual torque capacity is controlled under the secondary hydraulicpressure control. Further, the calculated torque capacity refers to atorque capacity during the returning process (phase (3) in FIG. 16) ofthe BSC (phase (2) in FIG. 16). The calculated torque capacity isspecifically a value based on or calculated from the actual secondaryhydraulic pressure and the actual gear ratio (torque capacity of one ofthe two pulleys 42, 43 to which engine torque is input, that is, theprimary pulley 42).

In step S521 a determination is made on whether or not the driverrequest torque is larger than the torque limit request from the BSC. Atthe result being YES, the flow proceeds to step S522 while at the resultbeing NO, the flow proceeds to step S525.

In step S522 following the determination on the driver request torque islarger than the torque limit request from the BSC in step S521, adetermination is made on whether or not the calculated torque capacityis larger than the torque limit request from the BSC. At the resultbeing YES, the flow proceeds to step S523 while at the result being NO,the flow proceeds to step S524.

In step S523 following the determination on the calculated torquecapacity>the torque limit request from the BSC in step S522, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value)+ΔT and the calculated allowabletorque capacity. The flow proceeds to RETURN.

In step S524 following the determination on the calculated torquecapacity≦the torque limit request from the BSC in step S522, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value) and the driver request torque. Theflow proceeds to RETURN.

In step S525 following the determination on the driver requesttorque≦the torque limit request from the BSC in step S521, adetermination is made on whether or not the calculated torque capacityis larger than the torque limit request from the BSC. At the resultbeing YES, the flow proceeds to step S527 while at the result being NO,the flow proceeds to step S526.

In step S526 following the determination on the calculated torquecapacity≦the torque limit request from the BSC in step S525, the torquelimit request from the BSC is set to a smaller one of the torque limitrequest from the BSC (previous value) and the driver request torque. Theflow proceeds to RETURN.

In step S527 following the determination on the calculated torquecapacity>the torque limit request from the BSC in step S525, the torquelimit request from the BSC is cancelled. The flow ends.

In the following the steps of the flowchart showing the transmissionrestricting process by limiting the target primary rotary rate in FIG.11 are described.

In step S541 a target inertia torque is calculated. The flow proceeds tostep S542.

In step S542 following the calculation of the target inertia torque instep S541, a target primary rotation change rate is calculated from thetarget inertia torque. Then, the flow proceeds to step S543.

In step S543 following the calculation of the target primary rotationchange rate in step S542, a limited target primary rotary rate notexceeding the target primary rotation change rate is calculated, and theflow proceeds to step S544.

In step S544 following the calculation of the limited target primaryrotation change rate in step S543, the transmission control is performedon the basis of the limited target primary rotary rate, and the flowproceeds to step S545.

In step S545 following the transmission control in step S544, adetermination is made on whether or not the transmission control basedon the limited target primary rotary rate is completed or the actualprimary rotary rate has reached the limited target primary rotary rate.At the result being YES (completion of transmission control), the flowends while at the result being NO (in the middle of transmissioncontrol), the flow returns to step S541.

Next, the operation of the control device and method for the belt typecontinuously variable transmission mechanism 4 according to the firstembodiment is described. It will be divided into tour parts, BSCpermission and continuation determining operations, BSC permission andcontinuation determining operations based on |command transmissionchange rate|<predetermined value, belt slip control operation (BSCoperation), and returning control operation from the BSC to the normalcontrol.

[BSC Permission and Continuation Determining Operations]

At a start of the vehicle's running, the operation proceeds to step S2from step S1 in the flowchart in FIG. 5. Unless all the BSC permissiondetermining conditions are satisfied in step S2, the flow from step S1to step S2 are repeated to continue the normal control. That is, thesatisfaction of all the BSC permission determining conditions in step S2is defined to be BSC control starting condition.

The BSC permission conditions in the first embodiment are as follows:

(1) The transmitted torque capacity of the belt type continuouslyvariable transmission mechanism 4 is stable (a change rate of thetransmitted torque capacity is small).

This condition (1) is determined by satisfaction of the following toconditions, for example.

-   a. |command torque change rate|<predetermined value-   b. |command gear ratio change rate|<predetermined value    (2) The estimated accuracy of the input torque to the primary pulley    42 is within a reliable range.    This condition (2) is for example determined on the basis of the    torque information (estimated engine torque) from the engine control    unit 88, the lockup state of the torque converter 2, the operation    state of a brake pedal, a range position and the like.    (3) The permitted conditions in the above (1) (2) are continued for    a predetermined length of time.    In step S2 whether or not the above conditions (1), (2), (3) are all    satisfied is determined.

Thus, the belt slip control is allowed to start if the transmissiontorque capacity of the belt type continuously variable transmissionmechanism 4 continues to be stable and the estimated accuracy of theinput torque to the primary pulley 42 is continuously within a reliablerange for a predetermined length of time during the normal control.

As above, the belt slip control is permitted to start upon thesatisfaction of all the BSC permission conditions so that it is able tostart the belt slip control in a preferable range with an assured highcontrol precision.

After the BSC permission is determined in step S2, in step S3 the beltslip control is performed to reduce an input to the belt 44 of the belttype continuously variable transmission mechanism 4 and maintain thebelt 44 in an appropriate slip state without slippage. Then, in step S4following the belt slip control in step S3, a determination is made onwhether or not all of the BSC continuation conditions are satisfied. Aslong as all of the BSC continuation conditions are satisfied, the flowfrom step S3 to step S4 is repeated to continue the belt slip control(BSC).

Here, the BSC continuation conditions are the BSC permission conditions(1), (2) and exclude the continuation condition for a predeterminedlength of time (3) of the BSC permission conditions. Because of this, itis made possible to prevent continuation of the belt slip control withunsecured control precision since the belt slip control is immediatelystopped and returned to the normal control if one of the conditions (1),(2) is unsatisfied during the belt slip control.

[BSC Permission and Continuation Determining Operations Based on|Command Transmission Change Rate|<Predetermined Value]

The belt slip control permission determination according to the firstembodiment is configured to permit the belt slip control under theconditions including one that the transmission speed as the change rateof a gear ratio is smaller than the predetermined value.

In other words, while the transmission change rate (range of change ingear ratio per unit time=transmission speed) is small, oscillationcomponents due to oscillation are generated during the transmission asshown in FIG. 12 showing the actual gear ratio characteristic relativeto the target gear ratio characteristic. However, at a smalltransmission change rate the gear ratio variation due to thetransmission and the oscillation component due to the oscillation can beseparated. That is, the accuracy of the belt slip condition estimatedfrom a monitored phase difference using the oscillation component of theactual gear ratio is high.

Meanwhile, while the transmission change rate is high, the oscillationcomponent included in the actual gear ratio disappears as shown in anarea C in FIG. 13 so that the gear ratio variation due to thetransmission and the oscillation component due to the oscillation cannotbe separated. That is, the accuracy of the belt slip condition estimatedfrom a monitored phase difference using the oscillation component of theactual gear ratio is low.

To the contrary, according to the first embodiment, at |command gearratio change rate|<predetermined value and at a high estimated accuracyof the belt slip condition, the belt slip control is permitted. Thisresults in reducing belt friction owing to a reduction in the secondaryhydraulic pressure and reducing a drive load on the transmissionmechanism by the reduction in the secondary hydraulic pressure. As aresult, the practical fuel efficiency of the engine 1 can be improved.

Meanwhile, at |command gear ratio change rate|≧predetermined value andat a low estimated accuracy of the belt slip condition, the belt slipcontrol is not permitted. This prevents the belt from greatly slippingduring the belt slip control, which occurs when the belt slip control ispermitted with no transmission speed condition satisfied. That is,during the belt slip control the clamp force of the belt is reduced dueto a reduction in the secondary hydraulic pressure. The belt with a lowclamp force may be greatly slipped with an increase in the input torqueto the transmission mechanism.

Next, concerning the BSC permission condition that |command gear ratiochange rate|<predetermined value, how to decide the predetermined valueas an upper limit for determining the magnitude of the command gearratio change rate is described. In the sine wave oscillation controller93 in FIG. 4, the belt slip control system is configured to superimposethe sine wave hydraulic pressure on the command secondary hydraulicpressure for the oscillation and estimate a belt slip condition from theoscillation component included in the actual secondary hydraulicpressure and that in the actual gear ratio Ratio. Because of this, it isa necessary condition for realizing the belt slip control that theoscillation component included in the actual secondary hydraulicpressure and that in the actual gear ratio Ratio are extracted to ensurethe estimated accuracy of the belt slip condition on the basis of theextracted oscillation component. In other words, the predetermined valueis set to a gear ratio change rate which is determined as a limit toallow the extraction of the oscillation component included in the actualsecondary hydraulic pressure and that in the actual gear ratio Ratio andthe ensuring of the estimated accuracy of the belt slip condition basedon the extracted oscillation components, with a gradual increase in thegear ratio change rate of the belt type continuously variabletransmission mechanism 4 during the belt slip control.

How to set this predetermined value is described in detail withreference to graph A in FIG. 14 showing the frequency characteristics ofthe gear ratio component and the oscillation component as well as tograph B in FIG. 14 showing the transmission condition when thetransmission range and transmission time constant are changed.

The transmission condition can be approximated to a time lag of firstorder and represented by a transmission range K and break frequency f(=1/T). With regard to the frequency characteristics in graph A in FIG.14, assumed that a different transmission range K1<K2 and a differentbreak frequency f1<f2<f3 are given. At the break f1 frequency thetransmission condition is {K1/(1+T1 s)} in graph B in FIG. 14, at thebreak frequency f2, it is {K1/(1+T2 s)} in graph B in FIG. 14, and atthe break frequency f3, it is {K1/(1+T3 s)} in graph B in FIG. 14. Thatis, the break frequency is an index for the transmission speed in acertain transmission range and the larger the break frequency, thefaster the transmission speed. Thus, with the transmission range K1 andthe break frequency f1<f2<f3 given, there is a margin to the limitfrequency D at which the command gear ratio component does not interferewith the oscillation component by the oscillation as shown in FIG. 14.In view of this, the upper limit threshold of the transmission speedwhen the transmission range is set to a maximum transmission range K2defined by the system is found to obtain the predetermined value. Forexample, with the transmission range set to K2 at the break frequency asshown in graph A in FIG. 14, the characteristics of the oscillationcomponent due to the oscillation and {k2/(1+T3 s)} interfere with eachother. This signifies that the transmission speed is over thepredetermined value. With the transmission range set to K2 at the breakfrequency f2 as shown in graph A in FIG. 14, the characteristics of theoscillation component due to the oscillation and {k2/(1+T2 s)} do notinterfere with each other and coincide at the limit frequency D. Thatis, the break frequency f2 is the upper limit break frequency whichdetermines the upper limit threshold of the transmission speed and thetransmission speed at the upper limit break frequency f2 is thepredetermined value. This signifies that there is a margin in thetransmission speed to the predetermined value when the break frequencyis smaller than the upper limit frequency f2 with the maximumtransmission range K2 set.

How to obtain the predetermined value as the upper threshold of thetransmission speed in reality is described. The profile of the commandgear ratio component has a constant characteristic (flat+constantgradient decrease characteristic), and the limit frequency D notinterfering with the gain characteristic of the oscillation componentand the maximum transmission range are known. The upper limit frequencyt2 is thus unambiguously decided from the known information so that thefrequency characteristic of the {k2/(1+T2 s)} can be drawn. Thetransmission speed is expressed by a rising gradient (=range of gearratio change per unit time) of the {k2/(1+T2 s)} characteristic shown ingraph B in FIG. 14, and this transmission speed is the upper limitthreshold (=predetermined value) for permitting the belt slip control.

Accordingly, by allowing the transmission speed (=gear ratio changerate) to be in the limit range in which the oscillation componentsincluded in the actual secondary hydraulic pressure and the actual gearratio Ratio are extracted, it is made possible to extend the range ofthe transmission speed in which the belt slip control is permitted whilethe estimated accuracy of the belt slip condition is ensured.

According to the first embodiment, the belt slip control is permittedwhen the command transmission change rate is lower than thepredetermined value. In other words, the permission for the belt slipcontrol is determined not from the actual gear ratio change rate of thebelt type continuously variable transmission mechanism 4 but from thetarget gear ratio decided by calculation. The permissions for startingand continuing the belt slip control are determined at the time when thecommand gear ratio change rate is calculated from a current gear ratioand the target gear ratio. Accordingly, it is able to determine thepermissions for starting and continuing the belt slip control on thebasis of estimated information such as the command gear ratio changerate before the gear ratio of the belt type continuously variabletransmission mechanism 4 actually changes.

[Belt Slip Control Operation (BSC Operation)]

At start of the belt slip control, the secondary hydraulic pressure isset to a value to acquire the clamp force not to cause belt slippagewith estimated safety so that the condition that the phase difference θis lower than the predetermined value 1 is satisfied. In the flowchartin FIG. 8 the flow from step S331→step S332→step S333→step S334→stepS335 to step S339 is repeated and every time the flow is repeated, thecommand secondary hydraulic pressure is decreased in response to thecorrection by −Δpsec. Then, until the phase difference θ at 1 or morereaches the predetermined value at 2, the flow proceeds from stepS331→step S332→step S333→step S334→step S336→step S337 to step S339 inFIG. 8 to maintain the command secondary hydraulic pressure. At thephase difference θ being the predetermined value at 2 or more, the flowproceeds from step S331→step S332→step S333→step S334→step S336→stepS338 to step S339 to increase the command secondary hydraulic pressurein response to the correction by +Δpsec. Under the belt slip control theslip rate is maintained so that the phase difference θ falls within therange of the predetermined values from 1 or more to less than 2.

The belt slip control is described with reference to the timing chart inFIG. 15. At time t1, the above BSC permission conditions (1), (2) aresatisfied and continued (BSC permission condition (3)). From time t2 totime t3, at least one of the above BSC continuation conditions (1), (2)becomes unsatisfied, and the BSC operation flag and SEC pressure F/Binhibiting flag (secondary pressure feedback inhibiting flag) are setfor the belt slip control. A little before time t3 the accelerator ispressed, so that at least one of the BSC continuation conditions becomesunsatisfied and the control to return to the normal control is performedfrom time t3 to time t4. After time t4, the normal control is performed.

Thus, as apparent from the accelerator opening characteristic, vehiclespeed characteristic, and engine torque characteristic as well as thesolenoid current correction amount characteristic of the secondaryhydraulic pressure solenoid 75 during steady running determinationindicated by the arrow E in FIG. 15, under the belt slip control thephase difference θ between the oscillation components of the secondaryhydraulic pressure due to the oscillation and the gear ratio ismonitored to increase or decrease the current value. Note that thesecondary hydraulic pressure solenoid 75 is normally open (always open)and decreases the secondary hydraulic pressure along with a rise of thecurrent value.

The actual gear ratio is maintained to be virtually constant by the beltslip control although it fluctuates with small amplitude as shown in theactual gear ratio characteristic (Ratio) in FIG. 15. The phasedifference θ, as shown in the phase difference characteristics of theSEC pressure oscillation and Ratio oscillation in FIG. 15, graduallyincreases with time from time t2 when the slip rate is approximatelyzero, and reaches a target value (target slip rate). The secondaryhydraulic pressure as shown in the SEC hydraulic pressure characteristicin FIG. 15 decreases with time from time t2 when safety is secured, asindicated by the arrow F, and reaches a value of the designed minimumpressure added with hydraulic pressure amplitude in the end which is inthe hydraulic pressure level with a margin to the actual minimalpressure. While the belt slip control continues for a long time, theactual secondary hydraulic pressure is maintained in the amplitude rangeof the designed minimum pressure plus hydraulic pressure to maintain thetarget value of the phase difference θ (of slip rate).

Thus, a decrease in the secondary hydraulic pressure by the belt slipcontrol results in reducing the belt friction acting on the belt 44 andreducing the drive load on the belt type continuously variabletransmission mechanism 4 by the reduction in the belt friction. As aresult, it is possible to improve the practical fuel efficiency of theengine 1 without affecting the travelling performance during the beltslip control based on the BSC permission determination.

[Returning Control Operation from BSC to Normal Control]

During the belt slip control while the BSC permission and continuationdeterminations are continued, the torque limit process in step S32 inFIG. 6 is performed by setting the torque limit request from the beltslip control as the driver request toque in step S321 in FIG. 7. In thefollowing torque limit operation for retuning to the normal control isdescribed with reference to FIG. 10 and FIG. 16.

The engine control unit 88 has a limit torque amount as an upper controllimit engine torque, and controls the actual torque of the engine 1 notto exceed the limit torque amount. This limit torque amount isdetermined according to various requests. For example, the upper limitinput torque to the belt type continuously variable transmissionmechanism 4 is set to the torque limit request during the normal control(phase (1) in FIG. 16), and the CVT control unit 8 sends the torquelimit request during the normal control to the engine control unit 88.The engine control unit 88 selects the minimum one of torque limitrequests from various controllers as the limit torque amount.

Specifically, at time t5 the phase (1) of the normal control is shiftedinto the belt slip control, and the torque limit request from the BSC issent to the engine control unit 88 in the phase (2) as shown in thelimit torque amount characteristic in FIG. 16. However, the torque limitrequest from the BSC during the BSC (phase (2) in FIG. 16) is forpreparation in advance for the torque limiting in FIG. 10 and does notvirtually function as a torque limit during the BSC (phase (2) in FIG.16).

Then, at time t6 the BSC continuation is aborted and shifted into thecontrol to return to the normal control. At time t6 a torque limitrequest is issued because of the driver request torque>torque limitrequest from the BSC and the calculated torque capacity≦torque limitrequest from the BSC. Therefore, the flow from step S521→step S522≦stepS524 to RETURN in the flowchart in FIG. 10 is repeated to maintain thetorque limit request from the BSC (previous value) in step S524.

Thereafter, at time t7 the driver request torque>torque limit requestfrom the BSC and the calculated torque capacity>torque limit requestfrom the BSC. The flow from step S521→step S522→step S523 to RETURN inFIG. 10 is repeated to gradually increase the torque limit request fromthe BSC to be (previous value+ΔT). Along with this rising gradient, theactual torque gradually rises.

Due to the rise of the torque limit request from the BSC since time t7,at time t8 the driver request torque≦torque limit request from the BSCand the calculated torque capacity>torque limit request from the BSC.The flow proceeds from step S521→step S525→step S527 to END in theflowchart in FIG. 10. In step S527 the torque limit from the BSC iscancelled.

In this example the flow skips step S526 which is executed when theaccelerator is manipulated as stepped on or returned (released) for ashort period of time. Specifically, step S526 is skipped when the beltslip control is cancelled by stepping-on of the accelerator and theaccelerator is released as soon as the returning control starts.

Owing to the torque limit control for limiting the changing speed of theinput torque to the belt type continuously variable transmissionmechanism 4 in returning from the belt slip control to the normalcontrol, it is possible to prevent the input torque to the belt typecontinuously variable transmission mechanism 4 from becoming excessivelylarge relative to the belt clamp force and prevent the belt 44 fromslipping.

Further, in the control to return to the normal control from the beltslip control, if the gear ratio of the belt type continuously variabletransmission mechanism 4 is changed at a general transmission speedwhile the changing speed of the input torque is reduced under the abovetorque limit control, a decrease in the input torque due to a change inthe rotary inertia occurs conspicuously. This may cause a driver to feelunnecessary deceleration (pull shock). In view of this, the changingspeed of the gear ratio is limited along with the limitation to thechanging speed of the input torque to the belt type continuouslyvariable transmission mechanism 4.

That is, upon the termination of the BSC continuation and shift to thecontrol to return to the normal control, the flow from step S541→stepS542→step S543→step S544 to step S545 in the flowchart in FIG. 11 isrepeated until completion of the transmission, to control thetransmission on the basis of the limited target primary rotary rate.

Thus, limiting the change rate of the primary rotation makes it possibleto reduce a change in the rotary inertia and prevent a reduction in theinput torque to the transmission mechanism. As a result, it is possibleto prevent a driver from feeling unnecessary deceleration (pull shock).

Next, the effects of the control device and method for the belt typecontinuously variable transmission mechanism 4 according to the firstembodiment are described in the following.

(1) The control device for the belt type continuously variabletransmission 4, including the primary pulley 42 for receiving an inputfrom a drive source (engine 1), the secondary pulley 43 for providing anoutput to the drive wheels 6, 6, and the belt 44 wrapped around theprimary pulley 42 and the secondary pulley 43, to control a gear ratiodetermined by a running radius ratio of the belt 44 on the pulleys bycontrolling a primary hydraulic pressure to the primary pulley 42 and asecondary hydraulic pressure to the secondary pulley 43, the controldevice further comprises a belt slip control means (step S3) configuredto oscillate the secondary hydraulic pressure and monitor a phasedifference between an oscillation component included in an actualsecondary hydraulic pressure and an oscillation component included in anactual gear ratio to estimate a belt slip condition, and control theactual secondary hydraulic pressure to decrease on the basis of theestimation to maintain to ensure a predetermined belt slip condition,and a belt slip control permission determining means (step S2)configured to permit the belt slip control means to perform the beltslip control when a change speed of the gear ratio of the belt typecontinuously variable transmission 4 is less than a predetermined valueat which the oscillation component due to the oscillation included in anactual gear ratio and a variation in the actual gear ratio can beseparated. Thus, it is able to provide the control device for the belttype continuously variable transmission mechanism 4 which can reducedrive energy consumption by a decrease in belt friction when anestimated accuracy of the belt slip condition is high and can preventthe belt 44 from greatly slipping during the belt slip control when theestimated accuracy of belt slip condition is low.(2) The belt slip control permission determining means (step S2)determines, on the basis of a gain characteristic of a gear ratiocomponent relative to a frequency and a gain characteristic of anoscillation component, an upper limit frequency which does not interferewith the gain characteristic of the oscillation component in a maximumrange of the gear ratio, calculates the change speed of the gear ratiofrom the determined upper limit frequency and the maximum range of thegear ratio, and sets the predetermined value of the change speed of thegear ratio to the calculated change speed of the gear ratio (graphs A, Bin FIG. 14). Thus, it is possible to increase the frequency and controlcontinuation time for the belt slip control during travelling bymaximally extending the range of permission conditions for the belt slipcontrol relative to the change speed of the gear ratio with an assuredestimated accuracy of the belt slip control.(3) The belt slip control permission determining means (step S2) isconfigured to permit the belt slip control means (step S3) to performthe belt slip control when a command transmission change rate is lessthan a predetermined value. Thus, it is able to determine the permissionfor starting the belt slip control on the basis of estimated informationsuch as the command gear ratio change rate before the gear ratio of thebelt type continuously variable transmission mechanism 4 actuallychanges.(4) A control method for a belt type continuously variable transmission4 by a belt slip control in which a belt slip condition among theprimary pulley 42, secondary pulley 43, and belt 44 is controlled with ahydraulic pressure, the method comprises the steps of oscillating thehydraulic pressure to control the hydraulic pressure on the basis of amultipled value of an oscillation component included in an actualhydraulic pressure and an oscillation component of an actual gear ratio,and permitting the belt slip control when a change rate of the gearratio of the belt type continuously variable transmission 4 is less thana predetermined value at which the oscillation component due to theoscillation included in an actual gear ratio and a variation in theactual gear ratio can be separated. Thus, it is able to provide thecontrol method for the belt type continuously variable transmissionmechanism 4 which can reduce drive energy consumption by a decrease inbelt friction when an estimated accuracy of the belt slip condition ishigh and can prevent the belt 44 from greatly slipping during the beltslip control when the estimated accuracy of belt slip condition is low.(5) In the belt slip control, the belt slip condition is estimated bymonitoring a phase difference calculated from the multipled value, tocontrol the hydraulic pressure on the basis of the estimation tomaintain a predetermined belt slip condition. Thus, it is possible tostably maintain a predetermined belt slip condition during the belt slipcontrol by accurately acquiring a change in the belt slip condition bymonitoring the phase difference correlated with the belt slip condition.As a result, under the belt slip control by which the belt friction isstably reduced, it is possible to realize a targeted reduction in driveenergy consumption.

Although the control device and method for the belt type continuouslyvariable transmission according to the present invention have beendescribed in terms of the exemplary first embodiment, they are notlimited thereto. It should be appreciated that design variations oradditions may be made without departing from the scope of the presentinvention as defined by the following claims.

The first embodiment has described an example where a hydraulic pressurecircuit of a single side adjusting type controlled by a step motor isused for the transmission hydraulic pressure control unit 7. However,another single side adjusting type or both sides adjusting typetransmission hydraulic pressure control unit can be also used.

The first embodiment has described an example where only the secondaryhydraulic pressure is oscillated. However, for example, the primaryhydraulic pressure together with the secondary hydraulic pressure can beconcurrently oscillated at the same phase by a direct acting controlsystem. Alternatively, the primary hydraulic pressure together with thesecondary hydraulic pressure can be oscillated at the same phase byoscillating the line pressure.

The first embodiment has described an example of an oscillation meanswhere the command secondary hydraulic pressure is given properoscillation components. Alternatively, solenoid current values can begiven proper oscillation components.

The first embodiment has described an application example of an enginevehicle incorporating a belt type continuously variable transmission.The present invention is also applicable to a hybrid vehicleincorporating a belt type continuously variable transmission, anelectric vehicle incorporating a belt type continuously variabletransmission and the like. In short it is applicable to any vehicleincorporating a belt type continuously variable transmission whichperforms a hydraulic pressure transmission control.

REFERENCE SIGNS LIST

-   1 engine-   2 torque converter-   3 forward/backward drive switch mechanism-   4 belt type continuously variable transmission mechanism-   40 transmission input shaft-   41 transmission output shaft-   42 primary pulley-   43 secondary pulley-   44 belt-   45 primary hydraulic pressure chamber-   46 secondary hydraulic pressure chamber-   5 final reduction mechanism-   6,6 drive wheel-   7 transmission hydraulic pressure control unit-   70 oil pump-   71 regulator valve-   72 line pressure solenoid-   73 transmission control valve-   74 decompression valve-   75 secondary hydraulic pressure solenoid-   76 servo link-   77 transmission command valve-   78 step motor-   8 CVT control unit-   80 primary rotation sensor-   81 secondary rotation sensor-   82 secondary hydraulic pressure sensor-   83 oil temperature sensor-   84 inhibitor switch-   85 brake switch-   86 accelerator opening sensor-   87 other sensors and switches-   88 engine control unit

The invention claimed is:
 1. A control device for a belt typecontinuously variable transmission, comprising a primary pulley forreceiving an input from a drive source, a secondary pulley for providingan output to a drive wheel, and a belt wrapped around the primary pulleyand the secondary pulley so as to control a gear ratio determined by arunning radius ratio of the belt on the pulleys by controlling a primaryhydraulic pressure to the primary pulley and a secondary hydraulicpressure to the secondary pulley, the device further comprising: a beltslip control means for oscillating the secondary hydraulic pressure andmonitoring a phase difference between an oscillation component includedin an actual secondary hydraulic pressure and an oscillation componentincluded in an actual gear ratio to estimate a belt slip condition, andcontrolling the actual secondary hydraulic pressure to decrease on thebasis of an estimation of the belt slip condition to maintain apredetermined belt slip condition; and a belt slip control permissiondetermining means for permitting the belt slip control means to performa belt slip control when a change speed of the gear ratio of the belttype continuously variable transmission is less than a predeterminedvalue at which the oscillation component included in the actual gearratio and a variation in the actual gear ratio are separated from eachother.
 2. The control device for a belt type continuously variabletransmission according to claim 1, wherein the belt slip controlpermission determining means permits the belt slip control means toperform the belt slip control when a command transmission change rate isless than a predetermined value.
 3. The control device for a belt typecontinuously variable transmission according to claim 1, wherein thebelt slip control permission determining means sets the predeterminedvalue of the change speed of the gear ratio to be in a limit range inwhich the oscillation components included in the actual secondaryhydraulic pressure and the actual gear ratio are extracted and anestimated accuracy of the belt slip condition based on the oscillationcomponents that are extracted is maintained.
 4. The control device for abelt type continuously variable transmission according to claim 3,wherein the belt slip control permission determining means permits thebelt slip control means to perform the belt slip control when a commandtransmission change rate is less than a predetermined value.
 5. Thecontrol device for a belt type continuously variable transmissionaccording to claim 1, wherein the belt slip control permissiondetermining means determines, on the basis of a gain characteristic of agear ratio component relative to a frequency and a gain characteristicof an oscillation component, an upper limit frequency which does notinterfere with the gain characteristic of the oscillation component in amaximum range of the gear ratio, calculates the change speed of the gearratio from the determined upper limit frequency and the maximum range ofthe gear ratio, and sets the predetermined value of the change speed ofthe gear ratio to the calculated change speed of the gear ratio.
 6. Thecontrol device for a belt type continuously variable transmissionaccording to claim 5, wherein the belt slip control permissiondetermining means permits the belt slip control means to perform thebelt slip control when a command transmission change rate is less than apredetermined value.
 7. A control method for a belt type continuouslyvariable transmission by a belt slip control in which a belt slipcondition among a primary pulley, a secondary pulley, and a belt iscontrolled with a hydraulic pressure, the method comprising: in the beltslip control, oscillating the hydraulic pressure to control thehydraulic pressure on the basis of a multiplied value of an oscillationcomponent included in an actual hydraulic pressure and an oscillationcomponent included in an actual gear ratio; and permitting the belt slipcontrol when a change speed of the gear ratio of the belt typecontinuously variable transmission is less than a predetermined value atwhich the oscillation component due to oscillation included in theactual gear ratio is separable from a variation in the actual gearratio.
 8. The control method for a belt type continuously variabletransmission according to claim 7, further comprising: in the belt slipcontrol, estimating a belt slip condition by monitoring a phasedifference calculated from the multiplied value and controlling thehydraulic pressure on the basis of an estimation of the belt slipcondition to maintain a predetermined belt slip condition.
 9. A controlmethod for a belt type continuously variable transmission by a belt slipcontrol in which a belt slip condition among a primary pulley, asecondary pulley, and a belt is controlled with a hydraulic pressure,the method comprising: in the belt slip control, oscillating thehydraulic pressure to control the hydraulic pressure on the basis of aphase difference between an oscillation component included in an actualhydraulic pressure and an oscillation component included in an actualgear ratio; and permitting the belt slip control when a change speed ofthe gear ratio of the belt type continuously variable transmission isless than a predetermined value at which the oscillation component dueto oscillation is extracted from a variation in the actual gear ratio.10. The control method for a belt type continuously variabletransmission according to claim 9, wherein the predetermined value ofthe change speed of the gear ratio is set to be in a limit range inwhich the oscillation components are extracted from the actual secondaryhydraulic pressure and the actual gear ratio.
 11. A control device for abelt type continuously variable transmission, comprising a primarypulley for receiving an input from a drive source, a secondary pulleyfor providing an output to a drive wheel, and a belt wrapped around theprimary pulley and the secondary pulley so as to control a gear ratiodetermined by a running radius ratio of the belt on the pulleys bycontrolling a primary hydraulic pressure to the primary pulley and asecondary hydraulic pressure to the secondary pulley, the device furthercomprising: a belt slip controller configured to oscillate the secondaryhydraulic pressure and monitor a phase difference between an oscillationcomponent included in an actual secondary hydraulic pressure and anoscillation component included in an actual gear ratio to estimate abelt slip condition, and control the actual secondary hydraulic pressureto decrease on the basis of an estimation of the belt slip condition tomaintain a predetermined belt slip condition; and a belt slip controlpermission determining unit configured to permit the belt slipcontroller to perform a belt slip control when a change speed of thegear ratio of the belt type continuously variable transmission is lessthan a predetermined value at which the oscillation component includedin the actual gear ration and a variation in the actual gear ratio areseparated from each other.
 12. A control device for a belt typecontinuously variable transmission according to claim 11, wherein thebelt slip control permission determining unit is configured to permitthe belt slip controller to perform the belt slip control when a commandtransmission change rate is less than a predetermined value.
 13. Acontrol device for a belt type continuously variable transmissionaccording to claim 11, wherein, the belt slip control permissiondetermining unit is configured to set the predetermined value of thechange speed of the gear ratio to be in a limit range in which theoscillation components included in the actual secondary hydraulicpressure and the actual gear ratio are extracted and an estimatedaccuracy of the belt slip condition based on the oscillation componentsthat are extracted is maintained.
 14. The control device for a belt typecontinuously variable transmission according to claim 13, wherein thebelt slip control permission determining unit is configured to permitthe belt slip controller to perform the belt slip control when a commandtransmission change rate is less than a predetermined value.
 15. Acontrol device for a belt type continuously variable transmissionaccording to claim 11, wherein the belt slip control permissiondetermining unit is configured to determine, on the basis of a gaincharacteristic of a gear ratio component relative to a frequency and again characteristic of an oscillation component, an upper limitfrequency which does not interfere with the gain characteristic of theoscillation component in a maximum range of the gear ratio, calculatethe change speed of the gear ratio from the determined upper limitfrequency and the maximum range of the gear ratio, and set thepredetermined value of the change speed of the gear ratio to thecalculated change speed of the gear ratio.
 16. The control device for abelt type continuously variable transmission according to claim 15,wherein the belt slip control permission determining unit is configuredto permit the belt slip controller to perform the belt slip control whena command transmission change rate is less than a predetermined value.